The present invention is particularly but not necessarily exclusively concerned with equipment in which such mechanisms operate in an environment in which finely divided solids are present, the solids being carried either in a fluid such as air (or other gas) or water (or other liquid). Such equipment includes screw conveyors, bucket elevators, ribbon mixers, so-called Z mixers, rotary valves, augers, pulverizers, hammer mills, powder transfer equipment and other mixing equipment. This list is not intended to be exhaustive and the invention could in principle be applied to any suitable equipment which operates in conditions in which it is desirable that gas or liquid borne solid material should be excluded from bearings or other components of the equipment, or should, for any other reasons, including environmental, health or safety reasons, be contained within a vessel or other enclosed space. Any such item of equipment is included in the term “applicable equipment” when it is used herein.
In the case of equipment comprising a rotatable member/aperture combination as described above, seals are used to prevent liquid, gas or solid matter (or a mixture thereof) from passing into or through the aperture. Most seals comprise a part (called a rotor) which is fixed on the rotatable member. Similarly, most seals comprise a part (called a stator) which is fixed on the stationary part of the mechanism.
Such seals must inescapably comprise an interface at which the rotor contacts or comes close to the stator and careful attention must be paid to the design of the components at the interface. Any leakage of the fluid medium in which the seal is working which takes place past the seal is most likely to occur at the interface. Also, a primary cause of failure of a seal is due to abrasive matter which is entrained in the fluid medium which penetrates between the stator and the rotor at the interface or the build up of heat due to rubbing contact between the stator and the rotor at the interface.
The present invention relates to so called labyrinth seals. This term is used loosely in the art but, for the purposes of this specification, a seal is one in which, by design, there is no intentional contact between the rotor and the stator at the interface. Rather, the parts of the rotor and stator are shaped so that there is a passage (or ‘labyrinth’) extending along the interface, the physical characteristics and the operating conditions therein being such as to effect the desired sealing action.
In conventional labyrinth seals which are designed for use with liquid media the passage is commonly so narrow that, by design, only a film of the liquid is present in the passage. This film is adherent to the surfaces of the stator and the rotor along the interface and it is the film which, in principle, prevents the liquid medium from passing through the passage when the seal is in use.
In conventional labyrinth seals which are designed for use with gaseous working media, the passage is commonly even narrower than the passage in a seal for liquids. The parts of the seal (including the parts of the stator and the rotor which form the passage at the interface) are arranged to bring about a pressure gradient between the ends of the passage which is sufficient to prevent the working medium from passing through the passage when the seal is in use.
Various means have been used to increase the effectiveness of labyrinth seals. For example, most seals for liquid working media have some means for removing any liquid which penetrates more than a certain distance into the passage. Many seals (commonly called dynamic seals), for both liquid and gaseous working media, have some means for raising the pressure in the passage (for example by pumping the liquid or gas into or through the passage) to a level which is higher than the ambient pressure of the medium in which the seal is working. The stators of yet other seals are provided with packings, brushes or the like which do in fact make contact the rotors in use. Packings are often used to prevent abrasive material carried in the working medium from penetrating to the interface. So-called brush seals are commonly used in turbine engines.
The conventional labyrinth seals of which the applicant is aware are intended for use in mechanisms where the shaft or other rotating member on which the rotor is mounted rotates at relatively high speed. In fact, in many cases, it is essential that such seals be operated at high speeds if they are to function property. By contrast, the shafts of applicable equipment generally operate at relatively low rotational speeds. For this reason, and also due to the fine clearance between the parts of the stators and the rotors of conventional labyrinth seals, the latter would be unsuitable to be used in applications where there is a substantial amount of abrasive material such as grit or dust in the working medium. There are however many industrial applications which call for such seals and it is well known that the seals which are conventionally used in such applications are not very satisfactory.
It is one object of the invention to provide a labyrinth seal which might find use in at least some applications in which the working medium is a dust producing pulverulent solid or a gas or liquid in which a substantial amount of finely divided solid material is suspended.